1. Field of the Invention
The present invention relates to an ink composition including at least two types of colored inks having different properties, a rewritable display medium utilizing the ink composition, and a method for displaying images.
2. Description of the Related Art
One known conventional rewritable display medium is a display medium in which an ink composition including a colorant is encapsulated and in which images are displayed and erased by moving the colorant. For example, Japanese Patent Application publication (JP-B) Nos. 51-10959 and 57-27463 and Japanese Patent Application Laid-Open (JP-A) No.2-193185 disclose display media in which magnetic particles are used as a colorant and in which images are displayed by applying a magnetic field to the magnetic particle to cause magnetic migration. In these display media, a dispersion of magnetic particles for display and masking particles having different colors is encapsulated and the magnetic particles are migrated by applying a magnetic field thereto so as to display and erase images.
However, this method has drawbacks: (1) a sharp image can not be displayed because the masking particles migrate along with the magnetic particles when displaying and erasing images and thus a mixture of those particles are present in image areas, and (2) images are erased incompletely and a background becomes dark because some magnetic particles surrounded by the masking particles can not migrate effectively when a magnetic field is applied.
JP-A Nos. 51-93827, 60-201378, and 4-58285 disclose a method in which a magnetic fluid is used in place of an ink and is caused to migrate. Referring to a magnetic fluid, Yamane has described in Proceedings of the Institute of Electrostatics of Japan Vol. 18, No. 6, p.491 (1994) that, in a magnetic fluid, superparamagnetic substances having mono magnetic domain structure and having particle diameters in the range from 10 to 100 nm are uniformly dispersed without aggregation as a result of addition of a surfactant.
JP-A No. 52-6376 discloses that, in the display medium utilizing the magnetic fluid, the magnetic fluid can be masked by another dispersion which does not dissolve the magnetic fluid, and can migrate to front of the display medium. However, the magnetic particles are eluted from the magnetic fluid over time because the surfactant adsorbed onto the particle surfaces is removed. In order to disperse particles, a great amount of the surfactant is usually added to the magnetic fluid used in the display medium. However, since the surfactant is physically absorbed on to the surface of the particles, the separation state is not permanently stable, and it is impossible to prevent magnetic particles and masking particles from being mixed with each other in the display medium. Further, since the magnetic particles in the magnetic fluid have small particle diameters, the magnetic fluid is somewhat transparent and has low coloring properties and therefore the display medium cannot provide high contrast.
In addition, a conventional display medium includes magnetic particles and masking particles dispersed in the same phase and mixed with each other. This creates that clear images having high contrast cannot be displayed.